1. Field of the Invention
The present invention relates to a composite encapsulating material, particularly a composite encapsulating material consisting of oxides.
2. Description of the Prior Art
In the fuel cell technology, Solid Oxide Fuel Cell (SOFC) has advantages such as high transfer efficiency, diversified fuel sources, and lower material cost. However, SOFC featuring high operating temperature (600° C.˜800° C.) demands a high-temperature encapsulation technology which is based on fluid glass-based encapsulating materials to fill gaps between SOFC and a substrate and insulate fuel gas from oxygen for airtight joint. For a fluid encapsulating material, glass transition temperature should be adjusted in the range of 600° C.˜800° C., operating temperature of SOFC.
In practice, materials used in SOFC are usually operated at high temperature. The materials experience cycles from to high temperature between normal startup and stop, so that some characteristics among different components such as matching attribute and heat stability are critical. Accordingly, an encapsulating material should be effective in a buffer action to reduce any stress-induced damage to one component out of expansion and contraction.
Shou-Guo Huang et al. disclosed Si—Al—Y—Ba—B glass in Journal of Alloys and Compounds wherein the ratios of various oxides are: silicon dioxide (3.53 wt %; 6.97 mol %), aluminum oxide (7.21 wt %; 8.39 mol %), yttrium oxide (10 wt %; 5.27 mol %), barium oxide (60 wt %; 46.55 mol %), and boron oxide (19.26 wt %; 32.83 mol %). As one material to encase connecting plates between components, this type of glass with boron and alkaline earth elements mixed may result in chemical compounds including cesium, magnesium, barium and calcium, which change an expansion coefficient and cause breaks between composite materials and connecting plates, as well as other chemical compounds on cathode of SOFC due to chemical reactions of alkaline earth elements deteriorating resistivity of SOFC operating at high temperature for a long time. On the other hand, heat stability of glass chronically used will be deteriorated in spite of lowered viscosity and increased wettability of boron oxide added glass due to addition of boron oxide which is characteristic of a low melting point and volatility but causes generation of boron hydroxide.